All-fibre devices for WDM optical communications

Abstract

This thesis is concerned with the study of two key technologies for enabling wavelength division multiplexed optical communication systems. The first is gain equalisation of the erbium-doped fibre amplifier and the second is the routing of optical channels through the network by means of all-fibre add-drop multiplexer configurations.
Firstly, in order to flatten dynamically the EDFA gain spectrum, an AO filter based on a multi-tapered fibre structure was demonstrated. The controlled taper profile was used as another degree of freedom for tailoring the filter loss spectrum. The coupling between the fundamental and several cladding modes was investigated by studying the evolution of the resonance conditions as the fibres are progressively tapered both theoretically and experimentally. The filter was demonstrated by equalising the EDFA gain spectrum for different saturation levels. The main advantage of this novel design when compared to alternative AO filters is its simplicity due to the reduced number of tuning parameters. Furthermore, a method of determining the ideal filter loss spectrum and correct placement within the amplifier was analysed. This is based on calculating the EDF wavelength dependent background loss necessary to equalise the amplifier gain spectrum, and integrating it into a discrete number of filters placed within the EDFA. Configurations based on one and two equalising filters were compared. Additionally, this method allowed novel complex filter designs, which could compensate for their own insertion losses as well as the insertion losses of other devices distributed along the amplifier, while achieving a flat gain spectrum.
Secondly, all-fibre OADMs based on the inscription of Bragg gratings in the waist of fused fibre-couplers were investigated. Design considerations of devices based on half- and full-cycle couplers were presented and their performances compared. In both these configurations the exact positioning of the gratings within the fused coupler waist is critical to achieve optimum performance. An all-fibre compact add-drop multiplexer based on a novel non-uniform half-cycle fused coupler is presented, providing an alternative OADM design with optimised symmetric operation, which is insensitive to the position of the grating in the coupler waist. The spectral performance of this 3cm long device is similar to that of a device based on a meter-long uniform half-cycle coupler. Finally, a technique for the nondestructive characterisation of couplers is proposed, in order to determine the 3dB points within the couplers waist. A CO₂ laser beam is scanned along the coupler length inducing a local perturbation to the coupler eigenmodes. Asymmetric and symmetric perturbations can give accurate mapping of power-evolution and coupler waist shape.

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